The IgE class of immunoglobulin is responsible for many of the clinical manifestations found in allergic patients. The overall objective of this proposal is to advance our understanding of the molecular mechanisms by which IgE-specific immune responses are regulated. In particular, we wish to determine the role of different sequences within the immunoglobulin heavy chain epsilon locus in the process of class switching to epsilon. During B cell development, class-switching allows a B lymphoid cell to change the function of the antibody it produces (which is determined by the heavy chain constant region of the molecule), which retaining the antigen binding specificity of the molecule (determined by the variable regions of the heavy and light chain). Studies have shown a correlation between the capacity of cytokines to modulate the isotypes of antibodies produced by B lymphoid cells with their capacity to affect the level of specific germline heavy chain transcripts. We have used gene targeting to determine if germline epsilon transcription is important in regulating class switching to IgE. The analysis of three different gene targeting experiments suggest that although germline epsilon transcripts is not sufficient to target the epsilon locus for high level class switching. This data contrasts with gene targeting experiments at the gamma 1 locus, which suggest that the production of germline gamma 1 transcripts is sufficient to target switching to IgG1. In this renewal application, we propose a series of experiments using the analysis of transgenic mice to further explore the role of germline epsilon transcripts and germline transcription int he targeting of immunoglobulin class switching to the epsilon locus. Specifically, we will: 1. Determine if the insertion of the E mu Vh element into the epsilon locus can target this region for high level class switching in splenic B cells; 2. Determine if the insertion of the metallothionein promoter and 3' end of the I epsilon exon into the epsilon locus can target this region for high level class switching; 3. Determine if expression of transgenic germline epsilon transcripts regulate class switching to IgE; 4. Determine the role of the S epsilon region in class switching to IgE.